Enum improvements

[mikeburke106]

This hotfix will fix any reported minor issues with RPC enum classes.

• Fix DisplayType valueForString throws a runtime exception #79
• Fix Enum valueForString methods are not consistent #91
• Fix ButtonName indexForPresetButton throws NullPointerException #102
• Fix SdlDisconnectedReason convertAppInterfaceUnregisteredReason throws NullPointerException #119
• Remove try/catch blocks when calling valueForString

[mikeburke106]

When this PR is merged, make sure to note #92 added valueOf() where it didn't exist before. Ensure VrCapabilities enum is updated with this change once #92 is merged as part of #113.

[mikeburke106]

Trivial change is being marked for 3-day offline review. Anticipated merge date is 2/23 at 3pm.

[mikeburke106]

Here are the results of the current unit tests for this PR.

Tests Run	Test Passes (%)	Test Failures (%)	Test Errors (%)
1486	1476 (99%)	9 (<1%)	1 (<1%)

Of the 10 failed tests, 9 are related to #71, and 1 is related to #89. Both of these issues are slated to be merged by version 3.3.0, so all unit tests should be passed by March 1 or earlier.