[layout precompilation] Add testing hook for precompiled layouts

We want to be able to control at a fine grained level whether precompiled
layouts are enabled so we can compare inflation results with and without. This
changes adds a @TestApi method that supports this.

Bug: 111895153
Merged-In: Ib6b62d79a9ca7aefefff8639752aa5838e491038
Change-Id: I2f28f6912499825e52c17cc163be9c0fe93855cb