Move non-treble devices to split file_contexts

This change is primarily to fix CTS which checks file ordering of
file_contexts. Having two separate means of loading file_contexts
has resulted in ordering variations.

Previously the binary file_contexts was preferred since it
loaded faster. However with the move to libpcre2, there is no
difference in loading time between text and binary file_contexts.
This leaves us with build system complexity with no benefit.
Thus removing this unnecessary difference between devices.

Bug: 38502071
Test: build and boot non-Treble Bullhead, run CTS tests below
Test: build and boot Treble Marlin, run CTS tests below
Test: cts-tradefed run singleCommand cts --skip-device-info \
    --skip-preconditions --skip-connectivity-check --abi arm64-v8a \
    --module CtsSecurityHostTestCases \
    -t android.security.cts.SELinuxHostTest#testAospFileContexts
Test: cts-tradefed run singleCommand cts --skip-device-info \
    --skip-preconditions --skip-connectivity-check --abi arm64-v8a \
    --module CtsSecurityHostTestCases \
    -t android.security.cts.SELinuxHostTest#testValidFileContexts
Change-Id: I088b3aeafaaab320f6658feb058a1fb89cbb65e1