build/make/core: Fix fragile assumptions about build toolchain.

- explicitly set the path where desugar will dump temporary class files.
  This ensures that the system property is set before it is read during
  InnerClassLambdaMetafactory.<clinit>. Before this CL, the system
  property was set by Desugar.createAndRegisterLambdaDumpDirectory(),
  which may run too late.

- explicitly specify -source 1.8 for droiddoc's javadoc run.
  Previously, the command used the language version of the build
  toolchain, which might fail due the backward-incompatible restrictions
  imposed by the proposed module system.

  Some Android build targets use LOCAL_JAVA_LANGUAGE_VERSION 1.7,
  but droiddoc combines sources from multiple build targets and there
  where no backwards incompatible changes in 1.8, so it should be fine
  to use 1.8 for code from either language level.

Bug: 38318052
Bug: 38225656
Bug: 38177295
Test: make clean && make checkbuild docs
      (using OpenJDK 8 toolchain)

Change-Id: Ie5fe0502dfe2f99fc58b1a723b7c18d5896a2646