Generalize hiddenAPIAugmentationInfo to make it easier to use

Previously, each category of flag file had its own property in the
hiddenAPIAugmentationInfo struct that required a lot of repetition
to use. This change moves the flag file specific handling into a
new hiddenAPIFlagFileCategory struct which allows use of the
hiddenAPIAugmentationInfo struct to be parameterized.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: I4413134c0c9382139bef3813f847e453f426692c