Don't merge apex variations with different UsePlatformApis

Merging apex variations with different UsePlatformApis results in
the presence of an extra apex with UsePlatformApis false changing
the behavior of an existing apex -> module dependency with
UsePlatformApis true, which can't be supported with incremental
analysis.  Don't merge apex variations with different UsePlatformApis
values, and add a _p suffix to the UsePlatformApis variation so they
can coexist.

Test: Test_mergeApexVariations
Bug: 372543712
Change-Id: I0e857a646a0449f87e4de6e6306ec921bc46fac1